Display device for motor vehicles

ABSTRACT

A display device (16) for motor vehicles features a graphic-capable and back-lighted transflective dot matrix LCD (19) the matrix dots of which can be activated electrically by way of driver stages, whereby they represent information (29) on the LCD by contrast in brightness between activated matrix dots (22) which have voltage applied to them and the other non-activated matrix dots. In order to obtain as contrasting as possible but dazzle-free information display in bright and in dark conditions, a contrast switch-over is effected on the LCD (19) in that the driver stages of the activated and the non-activated matrix dots can be switched over in dependence on the brightness in front of the LCD (19).

FIELD OF THE INVENTION

The present invention relates generally to display devices for motorvehicles and, more particularly, to a Liquid Crystal Display (LCD) forvehicular use in which either positive or negative contrast is used,depending upon ambient lighting conditions.

In known LCD display devices, the legibility of the display at differentdegrees of brightness in the surroundings in front of the display iskept as constant as possible, in that the brightness of the surroundingsis measured by a photo-electric detector, and the illumination of thedisplay device is made to follow by means of suitably dosed direct orindirect lighting of the display (EP-A3-011 55 75, Ziegler and Knoll,assigned to the assignee of the present invention). It is also knownthat the brightness of the display can additionally be controlled independence on the brightness in front of the vehicle in order thus totake into account the state of adaptation of the driver's eyes, which ismainly determined by the brightness in the driver's field of vision. Thelegibility of the displays however also depends on whether theinformation to be displayed is represented in positive contrast or innegative contrast. This applies particularly for graphic-capabletransflective dot matrix LCDs, on which information can be representedin the form of different symbols, figures, or numerical values and/orletters or words. In the case of a display in positive contrast wheredark symbols appear on a bright background, the legibility is good indaylight, while at night, due to the back-lighting of the LCD, the eyemust first adjust to the bright surroundings, which extends the readingperiod. With bright back lighting, there is the danger of dazzling thedriver, and with weak back lighting, the legibility is reduced by toolittle contrast. In the case of negative contrast, i.e. with brightsymbols on a dark background, the display is easily legible in the dark,as here the eye need not adapt to the background brightness. However, inthat case there is poor legibility in daylight, as shadows are formed bythe illumination on the background of the display, which shadows limitthe symbols to be shown. There occurs a so-called dazzle effect of theliquid crystal display.

In the present solution of the problem, the effort is made to optimizethe contrast of the display for graphic-capable transflective dot matrixLCDs as far as possible in dependence respectively on the lightconditions in the vehicle or in front of and behind the vehicle as thecase may be.

The Invention

The display device according to the invention has the advantage that,due to the contrast switch-over on the one hand a high contrast of thedisplay can be achieved during the day, in that when illuminated, thesymbols appear dark on a light background, and in that on the other handconversely in darkness the surroundings of the symbols are dark, so thatthe eye does not have to adapt to bright surroundings. Due to thissolution, the legibility of the display is no longer adversely affectedby light conditions outside or by back-lighting. It is to be regarded asa further advantage that the contrast switch-over is also independent ofwhether the LCD was made for positive or negative contrast. One cansimply invert the activation of the individual matrix dots of the LCDwithout altering the LCD, and thereby obtain a contrast switch-over. Inthe case of an LCD produced with positive contrast, the non-activatedmatrix dots are bright, i.e. translucent and become dark by the contrastswitch-over by applying a voltage, i.e. they are switched to block,whereas LCDs produced with negative contrast are dark when the matrixdots are not activated, and in the case of contrast switch-over areswitched to be bright by applying a voltage. In this solution it isfurther advantageous that the necessary light power for theback-lighting of the transflective dot matrix LCDs can be reduced.

The invention features a particularly effective and hence advantageouscontrast switch-over for the LCD, where a light sensor measures thelight density of the light falling on the LCD from the observation areaand controls the contrast switch-over of the LCD in dependence thereon.The electrical signal of the light sensor is usefully transmitted to athreshold switch, which with bright surroundings in front of the LCDswitches the information display to positive contrast and with darksurroundings switches it to negative contrast, The threshold switch canadvantageously have an adjustable switching threshold, which is to beindividually adjusted by the driver.

As, depending on the size of the dot matrix or the information to bedisplayed, the greater part of the matrix dots are either used torepresent the information or to form the surroundings of theinformation, it is advantageous if, regardless of the size and shape ofthe information displayed on the LCD, the greater part of the matrixdots are activated so as to be bright for bright surroundings in frontof the LCD, and the smaller part so as to be dark. Conversely, due tothe contrast switch-over, with dark surroundings the greater part of thematrix dots is activated so as to be dark and the smaller part so as tobe bright.

DRAWING

Embodiments of the invention are shown in the drawing and are explainedin greater detail in the following description. FIG. 1 shows a sectionof the vehicle with a display device built into the dashboard, and acontrast switch-over depending on the brightness. FIG. 2 shows a sectionof the display device according to FIG. 1 with a dot matrix LCD and FIG.3 shows sections of the dot matrix LCD with a) and b) in positivecontrast and c) and d) in negative contrast.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a sectional schematic representation of the driver area ofa motor vehicle 10 with a dashboard 11, which is situated behind orimmediately next to the steering wheel 12 of the motor vehicle 10.Therein light from the observation area 14 of a driver 15 falls onto thedisplay 16 of dashboard 11 in the direction of the arrow 13. In theembodiment, the display consists of a transflective liquid crystaldisplay (LCD). The display 16 is at the same time connected to anelectrical control circuit 17 which is supplied by an accumulatorbattery 18 of the motor vehicle 10. Measured data are transmitted to thecontrol circuit 17 from various sensors in the motor vehicle which arenot shown, and which are used to activate the corresponding displayelements on the LCD.

FIG. 2 shows a section of the display device 16 with a transflective dotmatrix LCD 19 in the dotted area next to various other dial displays 20.The dot matrix 19 consists of a multiplicity of picture elements, theso-called pixels, which are arranged in rows and columns and which canbe individually activated by driver stages which are not shown. Theelectrical driver stages are at the same time included in the controlcircuit 17 or arranged separately therefrom on the LCD. Depending on theconstruction of the dot matrix LCD 19, the light is allowed through oris blocked in the picture elements which have voltage applied to them.The blocked areas of the dot matrix 19 then appear dark, as neither isthe light falling on these areas from outside reflected there, nor isthe light from a light source 21 shown in FIG. 1 and arranged behind thedot matrix LCD 19 allowed to reach the outside in these areas. In thetranslucent areas of the dot matrix 19, on the other hand, light comingin from outside is for the most part reflected by a transflectorsituated behind the LCD or the light behind the LCD is allowed throughto the front. In this way there appears the information 29 consisting ofthe picture elements 22.

In FIG. 3 the contrast pictures of the dot matrix LCD 19 which arethereby possible are shown. FIG. 3a shows one picture element 22, whichis intended to show one piece of information in positive contrast, i.e.the picture element 22 appears dark against the bright background 23.The dot matrix LCD used in this case is so constructed that the light isblocked in the activated areas, i.e. those with voltage applied to them.In FIG. 3b on the other hand the light is conversely blocked in thenon-activated areas, i.e. in this case in picture element 22, while itis allowed to pass through in the activated areas. In this dot matrix,which is designed for negative contrast display, dark lines 24 thereforeappear between the activated picture elements 25 in positive contrast,in accordance with FIG. 3b. By switching over from positive contrast ofthe display in accordance with FIGS. 3a and 3b, to negative contrast inaccordance with FIGS. 3c and 3d, information representing pictureelement 22 appears on the other hand bright on a dark background 23. Inaccordance with FIG. 3c picture element 22 in the area of 19a of dotmatrix 19 is now not activated, while the other picture elements 25 areactivated, so that bright vertical and horizontal lines 24 becomevisible between them. In accordance with FIG. 3d, on the other hand, thearea 19b of an LCD designed for negative contrast, is only activated inrespect of picture element 22 and hence translucent.

In order to be able to detect the information appearing on the dotmatrix LCD 19 as quickly as possible, the contrast of the emittedinformation on its background must on the one hand be as good aspossible, and on the other hand the driver must at the same time not bedazzled. For this purpose it is provided in accordance with FIG. 1 thatthe light density of the light 13 falling on the display device 16 fromthe observation area 14 in motor vehicle 10 is detected by a lightsensor 27 via an optical fiber 26, and is converted into an electricalsignal in order to change over the driver stages of the respectivelyactivated and non-activated matrix dots 22 and 23 for the purpose ofeffecting the contrast switch-over of the dot matrix LCD 10 independence on the brightness in front of the LCD. For this purpose, inthe embodiment in accordance with FIG. 1, the light sensor 27 is aphoto-electric cell, which is connected to the input of a thresholdswitch 28 and whose signal intervenes in the control circuit 17 in sucha manner that the control signals for the individual picture elements ofdot matrix LCD 19 are inverted. The threshold switch 28 can be made as aSchmitt trigger with switching hysteresis. In the case of brightsurroundings in front of the LCD, this switches the displayedinformation over to positive contrast in accordance with FIGS. 3a and 3band in the case of dark surroundings to negative contrast in accordancewith FIGS. 3c and 3d. Due to the illumination from the observation space14 in vehicle 10 a good contrast is thereby obtained during the day bythe dark symbols 22 on the bright background 23 and at night by thebright symbols 22 on the dark background 23 due to the illumination fromthe light source 21 behind the dot matrix 19 to be switched onsimultaneously with the switch-over signal to give an easily legibledisplay without the danger of dazzle, since the eye does not have toadapt to a dark background.

Depending on the size and shape of the information to be indicated onthe LCD, more or fewer matrix dots 22 are required for this, Thus in thecase of relatively large information displays, the contrast can then notbe adversely affected by an insufficiently large background, and atnight there can thereby arise a danger of dazzle by a bright, relativelylarge information display. In order to take this into account also whenswitching over the contrast on the dot matrix LCD 19, it is possible tocompare the number of bright activated and dark activated matrix dotsfor each information display. Regardless of the size and shape of theinformation displayed on the LCD, in the case of bright surroundings infront of the LCD 19 the greater part of the matrix dots can thereby beactivated to be bright, while the smaller part is activated to be dark.Conversely, in the case of dark surroundings, the greater part of thematrix dots is activated to be dark to avoid the danger of dazzle andthe smaller part is activated to be bright. The threshold valuedepending on the brightness at which the automatic contrast switch-overtakes place, can be set by the manufacturer according to where thedisplay device is fitted or depending on the number, size and shape ofthe vehicle's windows, or it can be set individually, e.g. by hand bymeans of a potentiometer in the area of the display according to thedriver's requirements.

We claim:
 1. Display device for displaying a succession of images in amotor vehicle, comprisinga graphic-capable transflective dot matrix LCD(19); means (21) for backlighting said LCD, and a Liquid Crystal Display(LCD) control circuit (17), having an output connected to andcontrolling said LCD, said control circuit including driver stageselectrically activating matrix dots or pixels (22,25) of said LCD,memory means in said control circuit for representing information as oneof said images on said LCD by brightness contrast including instructionsto said drivers for applying a voltage to certain activated matrix dots,as opposed to the remaining non-activated matrix dots, to which novoltage is applied, means in said control circuit for comparing, in eachsuccessive image to be displayed on said LCD, the number of activatedmatrix dots or pixels to the number of non-activated matrix dots orpixels, in order to use the result of this comparison as a factor inautomatic selection of a contrast mode, and means (27) for selectivelysetting said display, under conditions of bright ambient lighting, to apositive contrast mode in which a majority of pixels in said image arebright, thereby assuring adequate legibility of said information, andunder conditions of dim ambient lighting, to a negative contrast mode inwhich a majority of pixels in said image are dark, thereby preventingdazzling of a vehicle operator.
 2. Display device in accordance withclaim 1, further comprisinga light sensor (27), having an outputconnected to an input of said display control circuit (17), whichmeasures the light density of light (13) falling on the LCD (19) fromoutside and, in dependence thereon, controls switching over of thecontrast of the LCD (19) from one of said modes to the other mode. 3.Display device in accordance with claim 1, further comprisingalight-dependent threshold switch (28), having an output connected to aninput of said display control circuit (17), which switches theinformation display to positive contrast in the case of brightsurroundings in front of the LCD (19) and to negative contrast in caseof dark surroundings.
 4. Display device according to claim 1, whereinswitchover from said positive contrast mode to said negative contrastmode occurs automatically when ambient light level sensed by a sensor(27, 28) drops below a predetermined threshold.
 5. Display deviceaccording to claim 1, whereinswitchover between said positive contrastmode and said negative contrast mode is controlled manually by a vehicleoperator.